At present, there have been various types of actuators, which have been widely used in various industrial fields. Further in recent years, because of the needs for energy saving and space saving, there is a strong need for a power-saving and compact size actuator.
As the power-saving and compact size actuator, there has been a so-called mini-actuator. This mini-actuator is manufactured by applying ultra-fine grinding to an inner side of an outer rail formed by drawing to provide a surface on which a ball moves and rotates and also by applying hybrid machining ball circulating machining for an inner block thereof and ball circulating machining for a ball screw nut. The height of a mini-actuator manufactured as described above is around 20 mm.
A servo motor is indispensable for an actuator based on the ordinary type of ball screw including the mini-actuator as described above. To reduce size of an actuator, also it is necessary to minimize the size of the servo motor. At present, compact servo motors each having a diameter of around 20 mm are commercially available.
Further there are various types of micro-actuators, which are important devices for micro-machines gathering hot attentions from the related circles. There are various principles for driving a micro-actuator including static electricity, a piezoelectric element, shape memory alloy, and thermal expansion. Generally, the size of a micro-actuator is in a range from 10 .mu.m to around 1 mm.
Also in recent years, as a means for minimizing a moving mechanism, there has been proposed an actuator based on an inch-worm system. FIG. 18 is a view showing an upper surface of an actuator based on the inch-worm system. In this figure, the reference numeral 802 indicates a guide rail. An H-shaped movable body 803 is provided between guide rails 802, 802. This movable body 803 comprises two holders 831, 832 each perpendicular to the guide rails 802, 802, and a holder section 833 in parallel to the guide rails 802, 802. Piezoelectric elements 834 to 846 are imbedded in the holder sections 831 to 833 respectively. PZT-based piezoelectric ceramics are used as piezoelectric elements 834 to 836. A side wall of each of the holder sections 831 to 833 is thin. The reason is that a space between the side walls can be expanded in association with extension of the piezoelectric elements. The table 804 is attached to an upper section of the movable body 803.
When a voltage is loaded to the piezoelectric element 835, the piezoelectric element 835 extends because of the piezoelectric effect. When the piezoelectric element 835 extends, the holder section 832 expands, and is fixed between the guide rails 802, 802. In this state, a voltage is loaded to the piezoelectric element 836. Then, the piezoelectric element 836 extends, and the holder section 833 expands. When the holder section 833 expands, the holder section 831 moves together with the piezoelectric element 834. Then, a voltage is loaded to the piezoelectric element 834 to expand the holder section 831. With this feature, the holder section 831 is fixed between the guide rails 802, 802.
Then, when loading of a voltage to the piezoelectric element 835 is stopped, the piezoelectric element 835 shrinks to the original size. With this, fixture by the holder section 832 is released. Then, when loading of a voltage to the piezoelectric element 836 is stopped, the piezoelectric element 836 shrinks to the original size, and the holder section 833 shrinks. In association with shrinkage of the holder section 833, the holder section 832 moves. In this actuator 800 based on the inch-worm system, the table 804 is moved by successively executing the sequence as described above.
Examples of various types of actuator were described above, but each of the actuators has problems as described below. At first, in the conventional type of actuator based on the ball-screw system, circulation of balls is required, and there is a limit in reducing the size because of its construction. Also as a servo motor is used, the power consumption is large. At the same time, the micro-actuator is too small in its size so that it is rather difficult to handle and can not practically be used for general industrial purposes.
The conventional type of actuator 800 based on the inch-worm system is smaller as compared to an actuator based on a ball-screw system. However, as a three-piece piezoelectric element is used, further reduction of the size is impossible, and a method of driving the actuator 800 itself is complicated.